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Abstract:

A hexapod system for aligning an optical element in semiconductor clean
rooms or in a vacuum, particularly in an illumination device for a
microlithographic EUV projection exposure apparatus, comprises six
hexapod supporting structures (42, 44, 46, 94; 42', 44', 46', 94', 106).
Using a set of at least two replaceable spacer elements (94; 94') having
a different extent in at least one direction, at least one of the six
supporting structures (42, 44, 46, 94; 42', 44', 46', 94', 106) can be
adjusted. The latter is adapted so that a spacer element (94; 94') can be
removed or a spacer element (94; 94') can be added while the coupling of
the first coupling end (46; 46') to the carrying structure (38; 38') and
the coupling of the second coupling end (54a; 54') to the optical element
(34; 34') are maintained. As an alternative, a hexapod system having six
hexapod supporting structures (42, 44, 46, 94; 42', 44', 46', 94', 106)
is provided, in which each supporting structure (42, 44, 46, 94; 42',
44', 46', 94', 106) comprises at least one monolithic articulation (60,
76; 60', 76'). By means of a set of at least two replaceable spacer
elements (94), the working length of at least one of the six supporting
structures (42, 44, 46, 94; 42', 44', 46', 94', 106) can be adjusted. A
method for aligning an optical element in semiconductor clean rooms or in
a vacuum by means of a hexapod system is furthermore provided.

Claims:

1. A damping device of an optical element of a projection exposure
machine, comprising: at least two mass dampers, arranged spaced apart
from one another, the mass dampers each having at least one damper mass
and at least one damping element, wherein the damper masses of the at
least two mass dampers are interconnected by at least one connecting
element.

2. The damping device of claim 1, wherein a mass of the connecting
element is less than the mass of a damper mass.

3. The damping device of claim 1, wherein the connecting element has a
stiffness sufficient to displace parasitic vibrational modes of the
vibration absorber into an uncritical range.

4. The damping device of claim 1, wherein at least two damper masses and
a connecting element are a conjoint component.

5. The damping device of claim 4, wherein the component is an annular
element.

6. The damping device of claim 4, wherein the component comprises a joint
that decouples a damper mass and an adjacent connecting element for a
vibration transmission.

7. The damping device of claim 6, wherein the joint is a solid joint that
corresponds to a reduction in the cross section of the component.

8. The damping device of claim 4, wherein the component is coupled to the
element to be damped at at least one bearing point.

9. The damping device of claim 1, wherein at least one of the mass
dampers has at least two damping elements which couple, the damper mass
to the element to be damped.

10. An optical element for a projection objective of a projection
exposure machine comprising the damping device of claim 1.

11. The optical element of claim 10, wherein the damping device is
fastened on the periphery of the optical element.

12. The optical element of claim 10, wherein the damping device is
fastened on the periphery of a holder accommodating the optical element.

13. A projection exposure machine with at least one optical element
according to claim 10.

14. A damping device of an optical element of a projection exposure
machine, comprising: at least two mass dampers, arranged spaced apart
from one another, the mass dampers each having at least one damper mass
and at least one damping element, wherein the damper masses of the at
least two mass dampers are interconnected by at least one stiff
connecting element.

15. A damping device of an optical element of a projection exposure
machine, comprising: at least two mass dampers, arranged spaced apart
from one another, the mass dampers each having at least one damper mass
and at least one damping element, wherein the at least one damper mass is
connected to the structure to be damped via the at least one damping
element and the damper masses of the at least two mass dampers are
interconnected by at least one connecting element such that the damper
masses provide mutual support.

16. The damping device of claim 1, wherein the at least one damping
element comprises a material selected from the group consisting of paper,
felt and an elastomer.

17. The damping device of claim 1, wherein the at least one damping
element comprises a fluoroelastomer.

18. The damping device of claim 1, wherein the at least one damping
element comprises a fluororubber.

19. The projection exposure machine of claim 13, wherein the at least one
damping element comprises a material selected from the group consisting
of paper, felt and an elastomer.

20. The projection exposure machine of claim 13, wherein the at least one
damping element comprises a fluoroelastomer.

21. The projection exposure machine of claim 13, wherein the at least one
damping element comprises a fluororubber.

22. The damping device of claim 15, wherein the at least one damping
element comprises a material selected from the group consisting of paper,
felt and an elastomer.

23. The damping device of claim 15, wherein the at least one damping
element comprises a fluoroelastomer.

24. The damping device of claim 15, wherein the at least one damping
element comprises a fluororubber.

25. The damping device of claim 16, wherein the at least one damping
element comprises a material selected from the group consisting of paper,
felt and an elastomer.

26. The damping device of claim 16, wherein the at least one damping
element comprises a fluoroelastomer.

27. The damping device of claim 16, wherein the at least one damping
element comprises a fluororubber.

[0002] The disclosure relates to a damping device of an optical element of
a projection exposure machine. The disclosure further relates to an
optical element for illuminating systems and projection systems of a
projection exposure machine or for systems in which an optical element is
exactly positioned with a damping device.

BACKGROUND

[0003] Projection exposure machines for semiconductor lithography are used
for the photolithographic production of semiconductor components and
other finely structured devices. In this case, the pattern of a mask
(reticle) is projected onto a substrate (wafer) usually coated with a
photosensitive layer. To this end, a projection exposure machine includes
a projection objective with a plurality of optical elements such as
lenses, mirrors, gratings or plane parallel plates, which are supported
via mounts in an objective housing of the projection objective. The
optical elements refract and/or reflect electromagnetic radiation emitted
by an illuminating device.

[0004] Mounting technology for optical elements of a projection objective
is disclosed, for example, in U.S. Pat. No. 6,229,657 B1. U.S. Pat. No.
6,229,657 B1 discloses a mount including an inner ring permanently
connected to the optical element, and an outer ring, the inner ring and
the outer ring being interconnected via joints, in particular via solid
joints, distributed on the periphery.

[0005] So-called manipulators can be used to compensate for aberrations.
Such manipulators include an optical element and adjusting elements which
are distributed on the circumference of the optical element. The
adjusting elements can be used to position and/or deform the optical
element. These manipulations of the optical element cause a change in the
imaging which compensates an aberration of the entire system. In some
embodiments, mounts for such manipulators include an inner ring or holder
with which the optical element is accommodated, and an outer ring or a
support device, it being possible to move the inner ring relative to the
outer ring using the adjusting elements.

[0006] The joints and/or the adjusting elements decouple the optical
elements, preferably from external structures, in terms of deformation.
The connections between the optical element and the external structures
preferably have a low stiffness.

[0007] In addition, imaging of ever smaller structures is desired in
microtechnology. Larger and therefore heavier optical elements can be
used for this. Both the low stiffness and the greater mass of the optical
elements respectively lower the natural vibration frequencies of the
system. This increases the risk of exciting the optical elements to
vibrations from outside.

[0008] It is therefore disclosed, for example, in WO 2006/084657 A1, to
provide a damper element between the outer ring and the inner ring.

[0009] Furthermore, it is disclosed in WO 2007/006577 A1 to provide a mass
damper with an additional mass or a plurality of additional masses on the
optical element, vibrational energy of the optical element being
dissipated by friction using the additional mass(es). Such an additional
mass is denoted below as damper mass. WO 2007/006577 A1 discloses fitting
an annular damper mass on an optical element or an inner ring using a
damper element. The damper element is arranged between the annular damper
mass and the optical element or the inner ring and is, for example, a
fibrous material such as paper, felt or an elastomer. Such mass dampers
are also denoted as annular absorbers. A good vibration damping can be
attained by means of annular absorbers for systems with one degree of
freedom.

SUMMARY

[0010] Damping devices for absorbing or dissipating the vibration energy
of an optical element of a projection exposure machine which is excited
to vibrations with a number of degrees of freedom are disclosed.

[0011] In general, in one aspect, the invention features, a damping device
of an optical element of a projection exposure machine, is provided which
includes at least two mass dampers arranged spaced apart from one
another, the mass dampers each having at least one damper mass and at
least one damping element, and the damper masses of the mass dampers
being interconnected by means of at least one connecting element.

[0012] The element to be damped can have an arbitrary three dimensional
restricted geometric form. For example, the element is fashioned in the
shape of a lens or disc.

[0013] As used herein, a "damping element" is designated as an element
which has spring and/or damping properties. A spring element can be
provided in a simple embodiment. The damping element may include a
material which dissipates vibrational energy, for example, paper, felt
and/or an elastomer. A fluoroelastomer, in particular a fluororubber, for
example Viton®, can have good properties for dissipating vibrational
energy. At the same time, a fluororubber may be suitable for application
in a projection objective because of the vacuum resistance and the good
thermal and UV resistance. Other elastomers are also advantageous
depending on application.

[0014] The natural frequencies of the mass dampers can be designed for a
frequency to be eliminated. Given this frequency, the mass dampers draw
vibrational energy from the structure, i.e., the optical element excited
to vibration. The mass dampers act in each case as segmented absorbers,
and so it is possible to damp a plurality of degrees of freedom. The
damper masses of the mass dampers are interconnected such that the damper
masses provide mutual support. In each case, mutually adjacent damper
masses are preferably interconnected via a connecting element or a
plurality of connecting elements. The support of the damper masses
eliminates parasitic solid-state modes such as can occur with
conventional segmented absorbers. In some embodiments, the individual
mass dampers are operationally connected to the element at attachment
points distributed discretely over the periphery of the element to be
damped. This renders it possible to target the placement of the mass
dampers at vibration excitation points and/or points of maximum vibration
amplitudes. In some embodiments, a plurality of discrete, i.e., spatially
distributed, attachment points are provided for a damper mass.

[0015] In certain embodiments, a mass of a connecting element is less than
the mass of a damper mass. As a result of this, the connecting elements
have a high natural frequency and are not excited to flexural vibrations
or are excited at least only in a high frequency range which is of little
relevance to the associated system, for example, in a range of greater
than 1,000 Hz. In addition, or as an alternative, the natural frequency
of the connecting element can be influenced by its stiffness. For
example, the connecting element stiffness can be selected so that the
connecting element displacesparasitic vibrational modes into an
uncritical range, for example, a range of greater than 1,000 Hz.

[0016] In some embodiments, at least two damper masses and a connecting
element are designed as a conjoint component, in particular as a
unipartite or integral component. The component can be fashioned as an
annular element, e.g., as a closed annular element. Such an annular
element can be arranged in a simple way on the rotationally symmetrical
element. Other configurations of the component can be provided which are
matched to the shape of the element to be damped. For example, a closed
annular element can be provided which has three damper masses and three
connecting elements arranged in an alternating fashion. It is usual for
three translational and three rotational vibrational modes to occur.
These can be effectively absorbed by three damper masses in conjunction
with a simple design of the system.

[0017] In general, the component preferably has at least one joint. The
joint decouples a damper mass and an adjacent connecting element for a
vibration transmission. Joints may be solid joints. A spot on a
unipartite component which has reduced flexural stiffness at least as
compared with the damper mass is denoted as "solid joint". A solid joint
can be implemented in a simple manner in a unipartite component. For
example, the solid joint may be provided as a reduction in the cross
section of the component, e.g., a discontinuous or sudden reduction in
the cross section. The reduction in the cross section can be provided in
such a way that the connecting element and the solid joint are fashioned
as segments of the component with the same cross section. In some
embodiments, the solid joint has a smaller cross section than the damper
mass and than the connecting element. A reduction in the cross section
can be implemented by a variation in the height of the annular element.
Alternatively, or in addition, a variation in the radius of the annulus
is provided. In some embodiments, the component includes a plurality of
discrete damper masses which are interconnected by means of spring
elements. The spring elements function in this case, for example, as
connecting masses and as joints. Alternatively, or in addition, a hinge
is provided.

[0018] The component can be coupled directly or indirectly to the element
to be damped at at least one bearing point. A further stabilization in
the x- and y-directions is attained owing to the bearing point. The
bearing point can be fashioned and/or arranged so as not to impair a
vibrational response in the z-direction of the damping device. In some
embodiments, the bearing point is arranged in a vibration node. The
bearing can be movable in the z-direction. In the case of an optical
element, the bearing point can be provided on an inner ring accommodating
the optical element such that an indirect attachment results.

[0019] In some embodiments, at least one of the vibration absorbers has at
least two or more damping elements. The damper mass can be coupled to the
element to be damped with the damping elements. The damper mass and the
damping elements may be tuned to one another so that the damper mass
constitutes a vibration form whose maximum vibration amplitude lies in
the region of the damping elements.

[0020] In some embodiments, an optical element for a projection objective
of a projection exposure machine is provided with the damping device. The
mass dampers, in particular the damper masses, the damping elements
and/or the attachment points can be optimally adapted in terms of their
arrangement, size and/or number to vibration forms of the element to be
damped. In other words, the vibration form of the damping device matches
in an adaptive fashion to the optical element, for example a manipulator.

[0021] The damping device can be fastened on the periphery of the optical
element. In some embodiments, the damping device is fastened on the
periphery of a holder accommodating the optical element. The holder is,
for example, the inner ring of a mount. In this case, the arrangement can
be suitably selected depending on the application.

[0022] In certain embodiments, a projection exposure machine with an
optical element is provided. It is possible in this case to provide an
optimum damping of vibration to avoid and/or reduce aberrations.
Vibrations can be eliminated by the damping device, and parasitic natural
frequencies of the mass damper can be shifted into ranges which are
considered harmless for the projection exposure machine Such harmless
frequency ranges can lie, for example, above approximately 1,000 Hz for
known projection exposure machines.

[0023] The above and other features will be apparent from the description,
the drawings and the claims. In general, features can be implemented
individually or in combination. Uniform reference symbols are used in the
drawings for identical or similar components.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In diagrammatic form, in the drawings:

[0025] FIG. 1: shows a top view of an optical element with a damping
device;

[0026] FIG. 2: shows a side view of the device in accordance with FIG. 1;

[0027] FIG. 3: shows a damping mass for a damping device in accordance
with FIG. 1;

[0028] FIG. 4: shows a damping element for a damping device; and

[0029] FIG. 5: shows a projection exposure machine

DETAILED DESCRIPTION

[0030] FIGS. 1 and 2 show diagrammatically an optical element 1 with a
mount 2 and a damping device. Here, FIG. 1 shows a top view, and FIG. 2 a
side view of the system. The optical element 1 is, for example, a lens
that is supported in a conventional way in a mount 2 comprising an inner
ring 20 and an outer ring 22. Various mounting technologies are
disclosed, for example, in U.S. Pat. No. 6,229,657 B1, the entire
contents of which is incorporated hereby by reference.

[0031] The inner ring 20 is rigidly connected to the optical element 1.
The inner ring 20 is mounted in the outer ring 22 via bearings 24. The
bearings 24 illustrated diagrammatically in FIGS. 1 and 2 are, for
example, fashioned as solid joints with a stiffness c and a damping d. It
is possible to provide between the outer ring 22 and the inner ring 20
further adjusting elements with a sensor system and/or actuator system
(not illustrated) which can be used to displace and/or deform the optical
element 1 actively by comparison with the outer ring 22.

[0032] Provided on the optical element 1 is a damping device with a
plurality of mass dampers 3 for the purpose of dissipating vibrational
energy. The damping device comprises at least two mass dampers 3
distributed over the periphery of the optical element 1. In the exemplary
embodiment illustrated in FIGS. 1 and 2, four mass dampers 3 are provided
which are distributed uniformly over the periphery of the optical element
1. In other refinements, more or less than four mass dampers 3 are
provided, it being possible for the mass dampers 3 to be distributed
uniformly or non-uniformly over the periphery of the optical element 1.

[0033] The mass dampers 3 comprise a damper mass 30 each. The damper
masses 30 are connected to the inner ring 20 and/or the optical element 1
via at least one damping element 32 or a plurality of damping elements
32, each. The mass dampers 3 are arranged on the inner ring 20 in the
exemplary embodiment illustrated diagrammatically in FIGS. 1 and 2. In
some embodiments, the mass dampers 3 are fitted directly on the optical
element 1, for example, being bonded thereto. As may be seen from FIG. 2,
the damping elements 32 are designed as spring damper elements in the
embodiment illustrated. The damping elements 32 of different damper
masses 30 are spatially separated from one another such that the damper
masses 30 are connected to the structure to be damped, for example the
optical element, at spatially separated attachment points.

[0034] As may be seen from FIG. 1, the individual damper masses 30 are
connected via connecting elements 4 whose mass is less than the mass of
the damper masses 30. The connecting elements 4 are illustrated
diagrammatically as stiff spring elements in FIG. 1. The stiffness of the
connecting elements 4 depends on a material used for this purpose, and on
the geometry, especially the cross-sectional area, of the connecting
elements 4. Because of the low mass of the connecting elements 4, the
connecting elements 4 are not excited to flexural vibrations or are
excited to flexural vibrations only in a very high frequency range. The
damper masses 30 are supported against one another in x- and y-directions
by the connecting elements 4. As a result, parasitic solid-state modes
such as can occur, for example, in the case of a damping device with
segmented absorbers known from the prior art generally do not arise.

[0035] The damper masses 30 can be optimally adapted to vibration forms of
the optical element in terms of their arrangement, size and/or number.
Alternatively or in addition, it is possible to influence the damping
behaviour by optimizing the number and/or arrangement of the damping
elements 32 and/or a variable of the damping elements 32.

[0036] FIG. 3 is a diagram of a component for an optimized damping device,
the damping masses 30 and the connecting elements 4 being designed as
integral parts of the component. The component is designed as a closed
circular ring, three damper masses 30 and three connecting elements 4
being arranged alternating with one another. The connecting elements 4
have a lesser height h by comparison with the height H of the damper
masses 30, and therefore a smaller cross section. The height H of the
damper masses 30 is, for example, approximately twice to eight times the
height h of the connecting elements 4. Because of the reduced cross
section, there is a reduction in stiffness in the transition between the
damper masses 30 and the connecting elements 4 such that a solid joint 34
results. Furthermore, constrictions are provided in the region of the
solid joints 34 in the embodiment illustrated. Because of the different
cross section, the mass of the damper masses 30 is higher than the mass
of the connecting elements 4. In some embodiments, a material with an
E-modulus of at least approximately 70×109 N/m2, in
particular of approximately 190×109 N/m2 can be used. It
is thereby possible to provide connecting elements 4 of adequate
stiffness using a comparatively small cross-sectional area and thus low
mass, such that the natural frequency of the connecting elements 4 is
very high and lies in a range causing no damage to the optical element.
The entire damper mass is preferably approximately 7% to approximately
12% of the mass of the optical element.

[0037] Furthermore, bearing points 36 are provided in the embodiment
illustrated for the purpose of supporting the damper masses 30. The
bearing points 30 are preferably situated in the region of vibration
nodes. In some embodiments, the bearing points 36 are provided in the
region of the connecting elements 4. In the embodiment illustrated in
FIG. 3, the damper masses 30 are attached to the inner ring 20 in
accordance with FIG. 1 using cylindrical damping elements 32, which are
also denoted as braids, and are illustrated schematically in FIG. 4. In
the embodiment illustrated, one damping element 32 is provided per damper
mass 30. In general, more than one damping elements per damper mass can
be provided. For example, two or more damping elements are provided per
damper mass 30.

[0038] FIG. 4 is a diagram of a damping element 32. The damping element 32
includes a layer 320 made from a material which dissipates vibrational
energy, being able to do so by friction. Paper, felt or the like are
examples in this case. In some embodiments, the material is an elastomer,
in particular a fluoroelastomer or fluororubber. Such an elastomer has
good characteristics which dissipate vibrational energy. At the same
time, a fluoroelastomer is suitable for application in a projection
objective. A disc, in particular a metal disc 322, 324, is respectively
provided on each side of the layer 320. The metal discs 322, 324 are
permanently connected to the elastomer 320, for example, by
vulcanization.

[0039] The following parameters, for example, are thus available for
setting the damping characteristics of the passive damping device: a
number and distribution of the mass dampers 3, a number of the damping
elements 32, a distribution of the damping elements 32 for each damping
mass 30, a thickness of the layer 320 of the damping elements 32, a
distribution of the damping masses 30 on the optical element 1, a mass of
the damping masses 30, a stiffness of the connecting elements 4 and/or a
mass of the connecting elements 4. It is thereby possible to match the
damping device 3 well to the optical element 1 and/or to an associated
manipulator. Consequently, a damping action can be greatly increased and
adapted to different vibrational modes.

[0040] FIG. 5 is a diagram of a projection exposure machine 5. The
projection exposure machine includes an illuminating device 50 with which
an illuminating radiation is produced, and a projection objective 51.
Arranged between the projection objective 51 and the illuminating device
50 is a mask 52 with a pattern, the pattern of the mask 52 being
projected onto a substrate 6 arranged beneath the projection objective
51. The projection objective 51 includes various optical elements 1 such
as lenses, mirrors, gratings or the like. In some embodiments, a
manipulator or a plurality of manipulators are provided to compensate
aberrations based on heating or the like. The manipulators respectively
include an optical element 1 in accordance with FIG. 1, and a sensor
system and/or actuator for positioning and/or deforming the optical
element 1.

[0041] The optical elements 1 can be supported in accordance with FIG. 1
using a mount comprising an inner ring 20 and an outer ring 22. The outer
rings 22 are accommodated in this case in a housing of the projection
objective 51. A damping device illustrated diagrammatically in FIGS. 1 to
4 may be provided on at least one of the optical elements 1 for the
purpose of absorbing vibrational energy.